It is known that certain electrochemically prepared semiconducting polymers such as polypyrrole may be employed in sensors in order to detect gases, vapors and odors. Such a sensor may comprise a pair of electrodes mounted on a substrate, with a layer of the semiconducting organic polymer deposited on and between the electrodes in order to produce an electrical connection between the electrodes. The semiconducting organic polymer may be sensitive to the presence of a gas or, more likely, to a range of gases, to the extent that adsorption of the gas onto the polymer surface affects the electrical properties of the polymer. Hence the presence of gas may be detected by monitoring, for example, the change in DC resistance of the sensor on exposure to the gas. For instance, Maisik et al (Maisik, J J, Hooper, A and Tofield, B C) JCS Faraday Trans. 1, 1986, 82, 1117-26 demonstrated a polypyrrole gas sensor displaying a DC resistance which was sensitive to the presence of nitrous oxide and hydrogen sulphide. GB-2,203,553-B discloses an improved method of detection wherein various AC impedance characteristics are measured at different AC frequencies.
A given semiconducting organic polymer will typically be sensitive to a range of compounds. Clearly this lack of selectivity is a major problem if one wishes to develop a sensor which is specific to a particular gas. Conversely, a sensor which employs a given semiconducting organic polymer may not be sufficiently sensitive to such a broad range of gases that it may be considered a general purpose device.